226 HYDRA 



ectoderm cells are both contractile and irritable, a special 

 degree of contractility is assigned to the muscle-processes 

 while the cells themselves are eminently irritable, the slightest 

 stimulus applied to them being followed by an immediate 

 contraction of the whole body. 



Imbedded in some of the large ectoderm cells are found 

 clear, oval sacs (A and B, ntc), with very well denned walls, 

 and called mmatocysts. Both in the living specimen and in 

 sections they ordinarily present the appearance shown in 

 Fig. 52, B, ntc, but are frequently met with in the condition 

 shown in E, that is, with a short conical tube protruding from 

 the mouth of the sac, armed near its distal end with three 

 recurved barbs, besides several similar processes of smaller 

 size, and giving rise distally to a long, delicate, flexible 

 filament. 



The examination of nematocysts in animals allied to, but 

 larger than Hydra, shows that the structure of these curious 

 bodies is as follows : Each consists of a tough sac, one end 

 of which is turned in as a hollow pouch : the free end of 

 the latter is continued into a hollow coiled filament, and 

 from its inner surface project the barbs. The whole space 

 between the wall of the sac and the contained pouch and 

 thread is tensely filled with fluid. When pressure is brought 

 to bear on the outside of the sac the whole apparatus goes 

 off like a harpoon-gun, the compression of the fluid forcing 

 out first the barbed pouch and then the filament, until finally 

 both are turned inside out. 



It is by means of the nematocysts the resemblance of 

 which to the trichocysts of Paramcecium (p. in) should be 

 noted that the Hydra is enabled to paralyze its prey. Pro- 

 bably some specific poison is formed and ejected into the 

 wound with the thread : in the larger members of the group 

 to which Hydra belongs, such as jelly-fishes, the nematocysts 



